The demand for transmitters and receivers of greater and greater data rates is forcing manufacturers to realize devices capable of working at higher frequencies with a broad range of adjustment of the frequency. Typically, these devices include an L-C resonant circuit, such as that depicted in FIG. 1, that amplifies an input signal at a certain frequency.
In order to fully exploit the advantages of working at a high frequency, it should be possible to tune these devices, i.e. to adjust the resonance frequency of the L-C resonant circuit. This may be done by including in the resonant circuit a plurality of inductors singularly connectable to the capacitor through selection switches, or by varying the capacitance through varactors.
The main limitation of monolithic implemented resonator-based circuits (e.g. L-C oscillators, filters) is a generally limited tuning range.
The resonant frequency variation attainable by integrated variable capacitors (i.e. MOS or PN junction varactors) is usually limited to about 20-30% for applications in the 1-10 GHz range. This restriction results from design trade-offs between tuning range, noise, phase accuracy (if quadrature is required) and power consumption. Process scaling toward the nanometer scale and the low supply voltage sources of portable applications like wireless devices, makes the problems in using integrated L-C circuits tuned in such a way more severe.
A different tuning approach can be followed based on a configuration with switched inductors. However, this approach is rarely followed because of the finite resistance of the selection switch when conducting worsens the quality factor figure of the resonant circuit.